Grinding Cermet Tool Materials
Cermet tools are generally used as indexable inserts, and the basic manufacturing steps are similar to cemented carbide tools. Grinding of the insert flank face and rake face is an important step in the fabrication of the tools. A considerable amount of knowledge exists in the industry relative to diamond characteristics and mesh size on how to grind different types of cemented carbides to maximize the grinding performance. However, the same parameters do not readily apply when grinding cermets, and very little published information is available to assist the tool fabricator.
For this reason, researchers at GE Superabrasives recently embarked on a study to examine the performance characteristics of two types of manufactured diamond that are widely used in resin bond systems to grind cermet materials. The results of the study offer some general guidelines on the effects of diamond friability and mesh size on the grindability of two different cermet materials. The results can also be applied to the grinding of technical ceramic components.
Test ProcedurePlunge reciprocating surface grinding tests were conducted to evaluate differences in diamond type, mesh size and wheel speed. Table 1 (p. 47) shows the grinding conditions used for testing. Both dry and wet grinding tests were initially considered, but researchers quickly discovered that dry grinding was not practical because it caused an extremely low grinding ratio and large wheel deflections.
The grindability of cermet materials was evaluated based on the normal grinding force, power consumption and grinding ratio (G) that could be used without creating edge chipping and microcracking. The surface finish (Ra) in the ground cermets was held between 0.25-0.4 µm. Two different cermet materials were used in this study, the difference between them primarily being alloying constituents that affect their hardness and transverse rupture strength. The average grain size of the carbides and nitrides in both these materials ranged from 1-3 microns. Figure 1 shows a comparison in friability of two types of diamond, RVG-W and RVG-890*, in the two mesh sizes used in this study. Since nickel coating is normally done on these products when they are used in a resin bond system, the friability data shown here corresponds to their respective uncoated diamond products. The relative friability values are normalized with respect to RVG-W diamond in a 140/170-mesh size.
Test ResultsThe effect of mesh size and diamond type was first evaluated on cermet KT175. Figure 2a shows the comparison in the normal force per unit width of the grind.
Since KT175 appeared easier to grind, researchers decided to examine the effect of increasing the wheel speed to 30 m/s and increasing the table speed to 15 m/min while keeping the depth of cut constant at 0.01 mm, using wheels containing RVG-W and RVG-890 in the 230/270-mesh size.
Finer vs. Coarser MeshSince cermet materials are known to have poor thermal properties relative to cemented carbides, they are easily susceptible to microcracking and chipping during grinding. Relatively gentle grinding conditions at low wheel speeds (about 3500-4500 sfpm) are therefore necessary to maximize wheel life. Wet grinding with a water-soluble oil coolant or a semi synthetic coolant is required to avoid low grinding ratios and large wheel deflections.
It also makes sense that a friable diamond, such as RVG-890, in a finer mesh size would be required to grind the cermet materials with low normal forces and power to produce good surface integrity. The test results verified this thinking in grinding the two different cermet materials.
However, a surprising observation in the test results was that for the range of mesh size used in this study, the grinding ratio was always higher for the finer mesh size (230/270) than for the coarser mesh (140/170) for both diamond types. This is in contrast to grinding cemented carbides, where it is generally observed that the grinding ratio and specific energies increase with coarser mesh size. This latter trend is also seen in grinding hardened alloy steels with cubic boron nitride (CBN), assuming that the surface finish and geometrical constraints are liberal. Since the material removal mechanism in all of these materials involves a plastic flow-type chip removal, the magnitude of the specific grinding energies and wheel wear strongly depends on the abrasive-workpiece interactions in a microscopic scale, as well as on the size of the chip that is generated in the process.
When grinding the cermet materials with a newly trued and dressed wheel containing 140/170 mesh, researchers observed that the grinding power and normal forces steadily increased with time. However, when using the wheel containing 230/270 mesh, the power and normal forces were high to begin with but quickly decreased with time to a steady state condition. Observations of the respective wheel surfaces under stereo microscope revealed that a heavy buildup of grinding detritus occurred in both the coarse and fine mesh wheels, but that the coarse mesh wheel exhibited a greater degree of bond degradation due to bond rubbing.
Based on these macroscopic observations, two possible explanations are proposed for the better performance of fine mesh diamond in grinding cermet materials. The first is based on the premise that for a constant diamond concentration in the wheel, material removal rate and wheel speed condition, the coarse mesh wheel will have fewer active diamond particles involved in the material removal process than the finer mesh wheel. The coarse mesh wheel will therefore have a greater probability to encounter the refractory carbides in the cermet in the course of a cut, while the fine mesh wheel will have a greater probability of penetrating the soft matrix and cradle the tough carbide in the chip formation process. The coarse mesh wheel thus expends a lot of energy in the plastic deformation or fracture of the refractory carbonitride constituents of the cermet, while the fine mesh wheel expends very little energy in removing the soft matrix material. The unit force per active diamond particle will therefore be much greater for the coarse mesh wheel and continue to increase with wheel wear.
The second explanation is based on the premise that the fine mesh wheel will have a greater number of diamonds engaged in the cut, increasing the real area of contact in the grinding zone and ensuring better heat transfer through the wheel. This has the potential to lower the temperature buildup in the part and the wheel, and thus reduce wheel wear.
While both of these explanations can also apply to grinding of cemented carbides and steel, the nature of abrasive-workpiece interaction is entirely different because of significant differences in the work material properties that alter the energy balance in the chip formation process.
Achieving Fine GrindingThe process parameters required to grind cermets efficiently differ considerably from the approach normally taken to grind cemented carbides. For the cermet materials used in this study, a fine mesh wheel containing the 230/270-mesh size produced significantly higher grinding ratio, lower power and lower normal forces relative to a wheel containing the 140/170-mesh size. This has more to do with the abrasive-workpiece interaction in the chip removal process, as explained earlier, than the relative friability differences in the diamond product. Edge chipping and microcracking of the cermet did not occur with the fine mesh wheel but was more obvious with the coarse mesh wheel.
A more friable diamond such as RVG-890 performed significantly better than RVG-W in the finer mesh size than in the coarser mesh size.
A low wheel speed of about 20 m/s (4000 sfpm) and wet grinding conditions were found to be effective in grinding the cermet materials. Dry grinding is not practical.
The process parameters developed to grind cermet materials can also be extended to grinding technical ceramic materials, such as silicon nitride, silicon carbide and AlTiC, that are applied in high-precision applications that require maintaining a good edge quality and fracture strength in the material. By using these same parameters, manufacturers can achieve the required integrity and performance in the ground ceramic part.
For More InformationFor more information about grinding cermets and other hard materials, contact GE Superabrasives at 6325 Huntley Rd., P.O. Box 568, Worthington, OH 43085; (614) 438-2000; fax (614) 438-2829; e-mail Carmen.Kassing@gep.ge.com; or visit http://www.AbrasivesNet.com.
*RVG-W and RVG-890 are trademarks of General Electric Co., U.S.A.